1. Field of the Invention
This invention relates to photography and, more particularly, to a structural arrangement for mounting a mirror in a photographic optical system.
2. Background of the Prior Art
Mirrors are widely used in photographic optical systems for a variety of reasons. The more important uses are to reduce the size of the system; to provide proper image of orientation; and to facilitate the viewing of a scene prior to selecting the subject matter of a picture. Systems that include a mirror as a part of them are conventionally referred to as reflex systems because the image formed by the focusing or objective lens is reflected off the mirror onto either a ground glass-screen for viewing or onto a photosensitive film unit. In either case the mirror is positioned in an oblique plane along and transverse to the optical axis of the objective lens in order to direct the bundle of rays emerging from the rear of the objective lens, by reflection, in another direction where they will be focused on the desired terminating plane of either the film or viewing screen. This redefinition of the path of the rays coming from the objective lens through reflection constitutes a sort of "folding" of the optical path of the system permitting the optical system to be packaged in a smaller space than would be required without " folding." The price for the reduction in size is, of course, the addition of another optical component to the system. This is true even where size is not the primary motive for the use of the mirror as, for example, in the case for proper image orientation. The reflex system is therefore relatively more complex than non-reflex systems and inherently has more problems associated with it. Included among these are the manner of spatially locating the oblique plane so that it will be in proper optical alignment with the other system components and the arrangement for mounting the mirror in that plane. In the prior art it is common practice to mount the mirror internal to the system exposure or viewing chamber by adhesively bonding it to one of the walls defining the chamber or to a carrier structure and then mounting that inside the chamber. There are several disadvantages to these approaches. Alignment becomes more difficult because of the number of parts involved and usually specialized fixturing is required in order to accurately place the mirror interior to the chamber or on the carrier member. In addition, it requires the selection of a suitable adhesive that will be compatible with the other materials and be capable of performing its function under a wide range of environmental conditions. However, even a suitable adhesive introduces problems because the mirror is usually unnecessarily restrained in such a way that its surface can be warped out of shape thereby degrading its optical qualities. The present invention, as will be seen, is directly concerned with the elimination of these problems.